The present invention relates to a headlight for a vehicle, and more specifically, to a vehicle headlight using, as its light source, a miniature high-pressure metal-vapor discharge lamp with an output of 100 W or less, such as a metal halide lamp, high-pressure sodium lamp, etc.
Conventionally, incandescent lamps have been used as a light source of headlights for vehicles. However, they have some drawbacks, including low luminous efficiency, short life, and need of frequent replacement.
On the other hand, discharge lamps are generally known as a light source with high luminous efficiency and long life. For example, fluorescent lamps, low-pressure discharge lamps, are used for interior illumination in buses, streetcars, etc. However, they are too bulky to be used for the light source of headlights.
In these circumstances, use of high-pressure metal-vapor discharge lamps for the light source of headlights is being studied. The discharge lamps of this type, which include metal halide lamps, high-pressure sodium lamps, and the like, are higher in luminous efficiency than fluorescent lamps, and can be miniaturized with ease. When using these discharge lamps for headlights, a battery or direct-current power source of 12 or 14 V, carried in a vehicle, is used for the power supply. Thus, the discharge lamps can be miniaturized for an output of 100 W or less, and the operating system is based on either the direct-current operating process or the high-frequency operating process. If the high-frequency process is used in operating such a discharge lamp, especially a metal halide lamp, however, an unstable wavelength range is wide, due to the influence of the metal sealed in the lamps. Thus, acoustic resonance is produced, which will prevent stable operation, possibly causing the lamp to go out. Accordingly, the discharge lamps of this type must be operated by using the direct-current operating process, in which the power supply undergoes no change of polarity.
When operating the metal-vapor discharge lamps by the direct-current process, however, color separation is liable to be caused by cataphoresis. This tendency is expressly marked if sodium is sealed in a luminescent tube. This is because sodium is so light, in weight, that it is drawn up to the side of a negative electrode, which constitutes the coldest region of the lamp, thus making the vapor-pressure distribution in the tube uneven. In conventional headlights, light emitted from the light source is radiated forward by a reflector. The aforesaid color separation causes a difference in the tone of color, between the central and peripheral portions of a luminous distribution pattern of a light beam, radiated from the reflector.
In view of the requirements of the recent car design, moreover, the headlights are generally expected to be thinner, or reduced in the vertical dimension. Preferably, therefore, the high-pressure metal-vapor discharge lamps, for use as the light source, should be not only miniaturized, but also arranged so that positive and negative electrodes are arranged horizontally inside the reflector, thus assuming a so-called horizontal operating posture. Horizontal operating, however, is very liable to cause cataphoresis, thereby accelerating the color separation in the luminous distribution pattern of the light beam.
In general, a long-wavelength light (reddish light) is higher in linearity than a short-wavelength light (bluish light). Thus, if the light beam, radiated from the headlight, undergoes light separation so that a reddish tint is intensive at its peripheral portion, the beam will inevitably disturb drivers of cars coming in the opposite direction.